Why Two?

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Anchor lead: Why do the Covid vaccines currently being used require two doses? Elizabeth Tracey reports

Most vaccines we’re accustomed to require a single injection, with a booster several years or even decades later. Why do the Covid vaccines currently in use require two doses? Andrew Pekosz, a vaccine expert at Johns Hopkins, explains.

Pekosz: With coronaviruses we have no immunity, so the first time your body sees this antigen, it recognizes it as foreign, but it doesn’t really go to the nth degree in terms of making antibody responses. It makes some, and actually this vaccine makes a significant amount, but it’s really the second time you see the antigen that your body now says, all right, this is foreign, and I’m seeing it again. Now this is really a threat so now I’m going to increase the amount of antibodies that I’m making and really generate a large strong amount of antibodies.  :30

Pekosz says most childhood vaccines do require a couple of doses, and the flu vaccine requires a single dose yearly because most of us have had some previous exposure to influenza. At Johns Hopkins, I’m Elizabeth Tracey.